Changeset 87e373b in mainline

Timestamp:

2011-01-13T21:18:25Z (14 years ago)

Author:

Jakub Jermar <jakub@…>

Branches:

lfn, master, serial, ticket/834-toolchain-update, topic/msim-upgrade, topic/simplify-dev-export

Children:

08abd81, c4fb95d3

Parents:

80cd7cd

Message:

Do not prevent new ARP requests to be made when the first one times out.
Add explanatory comments where useful.

File:

• uspace/srv/net/il/arp/arp.c (modified) (8 diffs)

uspace/srv/net/il/arp/arp.c

@@ -40 +40 @@
 #include <mem.h>
 #include <fibril_synch.h>
+#include <assert.h>
 #include <stdio.h>
 #include <str.h>
@@ -708 +709 @@
         bool retry = false;
         int rc;
+
+        assert(fibril_mutex_is_locked(&arp_globals.lock));
         
 restart:
@@ -725 +728 @@
         if (trans) {
                 if (trans->hw_addr) {
+                        /* The translation is in place. */
                         *translation = trans->hw_addr;
                         return EOK;
@@ -730 +734 @@
                 
                 if (retry) {
-                        /* Remove the translation from the map */
+                        /*
+                         * We may get here as a result of being signalled for
+                         * some reason while waiting for the translation (e.g.
+                         * translation becoming available, record being removed
+                         * from the table) and then losing the race for
+                         * the arp_globals.lock with someone else who modified
+                         * the table.
+                         *
+                         * Remove the incomplete record so that it is possible
+                         * to make new ARP requests.
+                         */
                         arp_clear_trans(trans);
                         arp_addr_exclude(&proto->addresses, target->value,
@@ -737 +751 @@
                 }
                 
+                /*
+                 * We are a random passer-by who merely joins an already waiting
+                 * fibril in waiting for the translation.
+                 */
                 rc = fibril_condvar_wait_timeout(&trans->cv, &arp_globals.lock,
                     ARP_TRANS_WAIT);
@@ -742 +760 @@
                         return ENOENT;
                 
+                /*
+                 * Need to recheck because we did not hold the lock while
+                 * sleeping on the condition variable.
+                 */
                 retry = true;
                 goto restart;
@@ -748 +770 @@
         if (retry)
                 return EAGAIN;
+
+        /*
+         * We are under the protection of arp_globals.lock, so we can afford to
+         * first send the ARP request and then insert an incomplete ARP record.
+         * The incomplete record is used to tell any other potential waiter
+         * that this fibril has already sent the request and that it is waiting
+         * for the answer. Lastly, any fibril which sees the incomplete request
+         * can perform a timed wait on its condition variable to wait for the
+         * ARP reply to arrive.
+         */
+
+        rc = arp_send_request(device_id, protocol, target, device, proto);
+        if (rc != EOK)
+                return rc;
         
         trans = (arp_trans_t *) malloc(sizeof(arp_trans_t));
@@ -763 +799 @@
         }
         
-        rc = arp_send_request(device_id, protocol, target, device, proto);
-        if (rc != EOK)
-                return rc;
-        
         rc = fibril_condvar_wait_timeout(&trans->cv, &arp_globals.lock,
             ARP_TRANS_WAIT);
-        if (rc == ETIMEOUT)
+        if (rc == ETIMEOUT) {
+                /*
+                 * Remove the incomplete record so that it is possible to make 
+                 * new ARP requests.
+                 */
+                arp_clear_trans(trans);
+                arp_addr_exclude(&proto->addresses, target->value,
+                    target->length);
                 return ENOENT;
-        
+        }
+        
+        /*
+         * We need to recheck that the translation has indeed become available,
+         * because we dropped the arp_globals.lock while sleeping on the
+         * condition variable and someone else might have e.g. removed the
+         * translation before we managed to lock arp_globals.lock again.
+         */
+
         retry = true;
         goto restart;